Nitride-based semiconductor substrate and semiconductor device

ABSTRACT

A nitride-based semiconductor substrate has a diameter of 25 mm or more, a thickness of 250 micrometers or more, a n-type carrier concentration of 1.2×10 18  cm −3  or more and 3×10 19  cm −3  or less, and a thermal conductivity of 1.2 W/cmK or more and 3.5 W/cmK or less. Alternatively, the substrate has an electron mobility μ [cm2/Vs] of more than a value represented by log e  μ=17.7−0.288 log e  n and less than a value represented by log e  μ=18.5−0.288 log e  n, where the substrate has a n-type carrier concentration n [cm −3 ] that is 1.2×10 18  cm −3  or more and 3×10 19  cm −3  or less.

The present application is based on Japanese patent application No.2005-350749, the entire contents of which are incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a nitride-based semiconductor substrate and,in particular, to a nitride-based semiconductor substrate that has ahigh thermal conductivity and electron mobility while securing asufficient electrical conductivity. Also, this invention relates to asemiconductor device fabricated using the nitride-based semiconductorsubstrate.

2. Description of the Related Art

Nitride-based semiconductor devices attract attention for a high-outputlaser diode (LD) used in a high-speed writing next-generation DVD, for ahigh-output light emitting diode (LED) used in a automobile headlight ora general lighting system, and for a high power conversion element. Insuch devices, fast dissipation of heat to be generated in high-outputoperation is a key issue so as to provide a high output, high efficiencyand high reliability device.

Thus far, the nitride-based semiconductor devices such as LED and LDhave been fabricated generally by growing epitaxial layers on a sapphiresubstrate by MOVPE etc.

However, since the sapphire substrate has a thermal conductivity as lowas 0.42 W/cmK, heat dissipation thereof is a serious problem. In thisregard, when a SiC substrate is used instead of the sapphire substrate,the heat dissipation property can be enhanced significantly since it hasa thermal conductivity of about 4 W/cmK. However, light extractionefficiency thereof is lowered since the conductive SiC substrate isthickly colored into green. In addition, dislocation density thereof isstill as high as the sapphire substrate.

A method of improving the heat dissipation property of LED is aflip-chip mounting that the epi-layer side of a LED chip is mounted on astem. However, when the sapphire substrate is used, a problem occursthat the amount of light incident to the sapphire substrate as a mainlight extracting portion does not increase as expected since there is abig refractive index difference between the epi-layer and the sapphiresubstrate. As a result, light extraction efficiency thereof is notenhanced so much. Furthermore, the fabrication cost must be increasedsince the process is complicated.

In case of using a GaN substrate, it is possible to provide a devicewith a good heat dissipation property due to its high thermalconductivity without reducing the light extraction efficiency.

A high-quality GaN is reported which has a thermal conductivity as highas about 2 W/cmK (See e.g., Document 1: D. I. Florescu et al., “Highspatial resolution thermal conductivity and Raman spectroscopyinvestigation of hydride vapor phase epitaxy grown n-GaN/sapphire(0001): Doping dependence”, Journal of Applied Physics 88(6) (2000) p3295). This value is about five times the sapphire (0.42 W/cmK) and is avery high value close to aluminum (2.4 W/cmK).

In general, it is necessary to secure a sufficient electricalconductivity in order to reduce the operating voltage of a device or toform an ohmic contact on the surface of a GaN substrate. Therefore,doping of impurity is needed so as to have a carrier concentration ofabout 1.2×10¹⁸ cm⁻³ or more.

However, in general, the thermal conductivity of a semiconductor crystallowers according to defect or impurity contained in the crystal. This isbecause phonon is dispersed by the defect or impurity or its complex.For example, Document 1 reports that the thermal conductivity of GaN isas high as 1.95 W/cmK at a carrier concentration (n) of 6.9×10¹⁶ cm⁻³,but it is reduced to about 0.5 W/cmK near at a carrier concentration (n)of 3.0×10¹⁸ cm⁻³. The latter thermal conductivity value is as low as thesapphire. Thus, the advantage of using the GaN substrate is spoiled.

In addition, since the impurity also causes the dispersion of carrier asdescribed earlier, carrier mobility may be lowered thereby. When thecarrier mobility is lowered, a high-density doping is needed to securethe same conductivity. This causes a vicious circle that the thermalconductivity lowers thereby. Thus, it is difficult to provide a GaNself-standing substrate with a high thermal conductivity and electronmobility when the carrier concentration is enhanced to secure asufficient electrical conductivity.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a nitride-basedsemiconductor substrate that has a high thermal conductivity andelectron mobility while securing a sufficient electrical conductivity.

It is a further object of the invention to provide a semiconductordevice using the nitride-based semiconductor substrate.

(1) According to one aspect of the invention, a nitride-basedsemiconductor substrate comprises:

a substrate comprising a nitride-based semiconductor,

wherein the substrate comprises a diameter of 25 mm or more, a thicknessof 250 micrometers or more, a n-type carrier concentration of 1.2×10¹⁸cm⁻³ or more and 3×10¹⁹ cm⁻³ or less, and a thermal conductivity of 1.2W/cmK or more and 3.5 W/cmK or less.

(2) According to another aspect of the invention, a nitride-basedsemiconductor substrate comprises:

a substrate comprising a nitride-based semiconductor,

wherein the substrate comprises a diameter of 25 mm or more, a thicknessof 250 micrometers or more, and an electron mobility μ [cm2/Vs] of morethan a value represented by log_(e) μ=17.7−0.288 log_(e) n and less thana value represented by log_(e) μ=18.5−0.288 log_(e) n, where thesubstrate comprises a n-type carrier concentration n [cm⁻³] that is1.2×10¹⁸ cm⁻³ or more and 3×10¹⁹ cm⁻³ or less.

(3) According to another aspect of the invention, a semiconductor devicecomprises:

the nitride-based semiconductor substrate as defined in (1) or (2); and

a nitride-based semiconductor layer grown epitaxially on thenitride-based semiconductor substrate.

In the above inventions (1) to (3), the following modifications andchanges can be made.

(i) The substrate comprises an electrical resistivity of 0.001 ohm·cm ormore and 0.02 ohm·cm or less.

(ii) The substrate comprises a dislocation density of 1×10⁷ cm⁻² orless.

(iii) The substrate comprises a concentration of As, O, Cl, P, Na, K,Li, Ca, Sn, Ti, Fe, Cr and Ni to be all 1×10¹⁷ cm⁻³ or less.

BRIEF DESCRIPTION OF THE DRAWINGS

The preferred embodiments according to the invention will be explainedbelow referring to the drawings, wherein:

FIG. 1 is a graph showing a relationship between a stay time over Gamelt of HCl gas and carrier gas and an impurity concentration in growncrystal;

FIG. 2 is a graph showing a relationship between a hydrogen partialpressure in growing by HVPE a nitride-based semiconductor on anunderlying substrate with a nano-mask formed thereon and an oxygenimpurity concentration in the grown crystal;

FIG. 3 is a graph showing a relationship between a rate of the thicknessof a facet-grown layer in the entire thickness of a GaN layer and anoxygen concentration in the grown crystal;

FIG. 4 is a schematic diagram illustrating an HVPE reactor used inExample 1 according to the invention;

FIGS. 5A to 5G are cross sectional views showing a method of making aGaN self-standing substrate in Example 2 according to the invention;

FIGS. 6A to 6F are cross sectional views showing a method of making aGaN self-standing substrate in Comparative Example 1;

FIG. 7 is a graph showing a relationship between a carrier concentrationand a thermal conductivity in Example 3 according to the invention;

FIG. 8 is a schematic cross sectional view showing an LED structure inComparative Example 2;

FIG. 9 is a schematic cross sectional view showing an LED structure inComparative Example 3;

FIG. 10 is a schematic cross sectional view showing an LED structure inComparative Example 4;

FIG. 11 is a schematic cross sectional view showing an LED structure inExample 4 according to the invention;

FIG. 12 is a graph showing an operating current dependency of opticaloutput in the LED's of Example 4, Comparative Examples 2-4;

FIG. 13 is a graph showing an operating time dependency of relativeoutput in the LED's of Example 4, Comparative Examples 2-4; and

FIG. 14 is a graph showing a relationship between a carrierconcentration and an electron mobility in Example 5.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A GaN self-standing substrate and a method of making the same inpreferred embodiments of the invention will be detailed below.

Size of Substrate

The GaN self-standing substrate of the embodiment has a diameter of 25mm or more and a thickness of 250 μm or more. If less than 25 mm indiameter, the productivity of the substrate lowers. If less than 250 μmin thickness, the mechanical strength of the substrate lowers and,therefore, the substrate may be hard to handle.

Thermal Conductivity

The GaN self-standing substrate of the embodiment has a thermalconductivity of 1.2 W/cmK or more when its n-type carrier concentrationis 1.2×10¹⁸ cm⁻³ or more.

In general, in order to secure an electrical conductivity of substrate,it is necessary to increase its carrier concentration by doping animpurity such as Si and Ge. However, due to the doping of the impurity,the thermal conductivity lowers. In this regard, the GaN self-standingsubstrate of the embodiment can succeed in reducing significantly theimpurity level by a method as described later (e.g., the concentrationsof As, O, Cl, P, Na, K, Li, Ca, Sn, Ti, Fe, Cr and Ni can be all reducedto 1×10¹⁷ cm⁻³ or less). Therefore, even when the carrier concentrationis increased by doping to have a sufficient electrical conductivity(i.e., even when the n-type carrier concentration is 1.2×10¹⁸ cm⁻³ ormore), the thermal conductivity can be 1.2 W/cmK or more, preferably 1.5W/cmK or more, more preferably 2.0 W/cmK. The reason for setting thelower limit of the thermal conductivity to be 1.2 W/cmK is that the heatdissipation effect of the device is insufficient if less than 1.2 W/cmK.

On the other hand, the upper limit of the thermal conductivity is to be3.5 W/cmK. This is because it is beyond a theoretical limit so that thecrystal may have some abnormality if more than 3.5 W/cmK. For example,anisotropic thermal conduction in superlattice structure may begenerated due to a periodical fluctuation in growth condition, whereby alarge thermal conductivity may appear in a specific direction and only asmall thermal conductivity may appear in a direction perpendicular tothe specific direction. As a result, heat will be confined locally.Further, even the normal epitaxial growth may be not performed since thecrystal structure is subjected to some abnormality.

Electron Mobility

The GaN self-standing substrate of the embodiment has an electronmobility, μ [cm²/Vs], of more than a value represented by log_(e)μ=17.7−0.288 log_(e) n and less than a value represented by log_(e)μ=18.5−0.288 log_(e) n, where the n-type carrier concentration n [cm⁻³]is 1.2×10¹⁸ cm⁻³ or more and 3×10¹⁹ cm⁻³ or less. The lower limit of theelectron mobility is determined in view of a balance between a dopingamount needed to have a desired carrier concentration and a reduction inthermal conductivity caused thereby. The upper limit of the electronmobility is determined in view of avoiding the anisotropic thermalconduction due to the superlattice structure and a biased flow ofcurrent accompanied therewith, or the incidence of epitaxial growthabnormality due to the abnormal crystal structure as in the case of thethermal conductivity.

Carrier Concentration

The GaN self-standing substrate of the embodiment has a n-type carrierconcentration n [cm⁻³] of 1.2×10¹⁸ cm⁻³ or more and 3×10¹⁹ cm⁻³ or less.If less than 1.2×10¹⁸ cm⁻³, it is difficult to form the ohmic contact tothe substrate so that the resistivity of the device cannot be reduced.If more than 3×10¹⁹ cm⁻³, the thermal conductivity lowers abruptly andthe normal epitaxial growth may be not performed due to a large strainapplied to the crystal.

The doped impurity (=dopant) can be Si, Ge, O, C etc. and a combinationthereof.

Electrical Resistivity

The GaN self-standing substrate of the embodiment has preferably anelectrical resistivity of 0.02 ohm·cm to secure the sufficientconductivity. When the doping amount is increased too much to reduce theelectrical resistivity (or to increase the conductivity, the thermalconductivity may be affected badly or the crystalline quality maydeteriorate. Therefore, the doping amount is preferably controlled so asto have a lower limit of 0.001 ohm·cm in electrical resistivity.

Method for Reducing the Impurity Concentration

It is important to reduce the impurity concentration in order to obtainthe abovementioned GaN self-standing substrate with the high thermalconductivity. In general, in order to reduce the impurity concentration,it is needed to increase the purity of a raw material, carrier gas and amember used in the growth, or to install a glove box or to conduct asufficient purge before the growth, or to make a complete control offurnace impurity level such as pre-baking.

However, a residual impurity incorporated in the grown crystal causes areduction in thermal conductivity.

The inventor considers how to further reduce the impurity level toenhance the thermal conductivity. As a result, the inventor finds thefollowing three effective methods (a) to (c)

(a) To Contact Source Gas or Carrier Gas with Ga Melt for a Long TimePeriod in the HVPE Method

To increase the purity of the raw material is fundamental to enhancementof the purity of crystal. In the HVPE method, GaCl as a group III sourceis generated by flowing or bubbling HCl gas and carrier gas over or inthe Ga melt disposed in the furnace to react them each other. The HClgas has a very strong corrosive property and contains many kinds ofimpurities, e.g., metal elements. The Ga melt has a high impuritytrapping effect. The inventor investigated the relationship between atime period to allow HCl gas to stay over (or contacted with) the Gamelt and a resultant impurity concentration in the grown crystal. As aresult, it is found that the purity of the grown crystal can besignificantly enhanced by setting the stay time period to be longer thana conventional time period.

FIG. 1 is a graph showing a relationship between the time period toallow HCl gas and carrier gas to stay over the Ga melt and theconcentration of impurity trapped in grown crystal.

As a result, it is understood that, by setting the stay time over the Gamelt to be about 1 min. (=60 sec.) or more, the oxygen (O) impurityconcentration can be significantly reduced to about 10¹⁶ cm⁻³ from about10¹⁸ cm⁻³ and the iron (Fe) impurity concentration can be significantlyreduced to about 10¹⁴ cm⁻³ from about 10¹⁷ cm⁻³. In general, the HCl gasis supplied into the reactor together with the carrier gas. Therefore,it is assumed that the carrier gas can be purified as well as the HClgas by being contacted with the Ga melt with the high impurity trappingeffect for 1 min. or more. However, it is impossible to purify NH₃ as agroup V source as well since the NH₃ reacts strongly with Ga.

Although the stay time needed over the Ga melt may vary depending onother factors such as a shape of reservoir, the same tendency as shownin FIG. 1 can be obtained even when the other factors vary. Thus, thestay time is to be about 1 min. or more.

(b) To Form a Thin Film with Microscopic Pores to Have the ImpurityTrapping Effect on an Underlying Substrate

The inventor finds that the thin film with a number of microscopic poresmade of a metal or metal compound is effective in preventing an impurityleft even after conducting the above process (a).

For example, after a nitride-based semiconductor layer is grown on asapphire substrate as the underlying substrate by MOVPE etc., a metalfilm made of titanium, nickel, tantalum, tungsten etc. is formed on thenitride-based semiconductor layer. Then, by conducting a heat treatmentin an atmosphere containing hydrogen and ammonia, the metal is nitridedand aggregated to provide a structure (herein called nanomask) with anumber of microscopic pores. The nanomask allows the relaxation ofstrain caused by lattice mismatch or a difference in thermal expansioncoefficient between the sapphire substrate and the nitride-basedsemiconductor layer so as to reduce the defect density (e.g., up to adislocation density of 1×10⁷ cm⁻² or less). Especially when the metalfilm of titanium is formed thereon and nitrided in surface, the titaniumnitride also serves as a buffer layer for the nitride-basedsemiconductor layer which is grown on the metal film. Therefore, thegrown nitride-based semiconductor layer can have a good crystallinequality.

The metal film can be grown by the vapor deposition method, sputtering,various CVD methods etc. The microscopic pores are desirably disperseduniformly on the surface of the metal film so as to reduce the defectdensity (e.g., up to a dislocation density of 1×10⁷ cm⁻² or less) in thenitride-based semiconductor layer to be grown thereon. The production ofthe microscopic pores can be controlled by the thickness of the metalfilm, the thickness of the nitride-based semiconductor layer on thesapphire substrate, and the conditions of the heat treatment. Forexample, in order to allow the nitriding of the metal film and 100 nm orless of uniform microscopic pores, the heat treatment is desirablyconducted at 700 degree Celsius or more and 1400 degree Celsius or less.If less than 700 degree Celsius, the nitriding is not sufficientlyadvanced and the uniform pores can be generated. If more than 1400degree Celsius, the heat decomposition of the nitride-basedsemiconductor may be excessively advanced so that the nitride film issubjected to separation. The thickness of the metal film is desirably 1micrometer or less. If more than 1 micrometer, the surface of the metalfilm may be formed uneven when the metal film is nitrided. This maycause an increase in defect in the nitride-based semiconductor layer tobe grown thereon.

The heat treatment can be in a hydrogen gas atmosphere or a mixed gasatmosphere containing hydrogen. The mixed gas atmosphere containinghydrogen is, for example, a mixed gas atmosphere of 80 to 60% ofhydrogen gas and 20 to 40% of ammonia.

FIG. 2 is a graph showing a relationship between a hydrogen partialpressure in growing by HVPE a nitride-based semiconductor on anunderlying substrate with a nanomask formed thereon and an oxygenimpurity concentration in the grown crystal.

As a result, it is understood that, by setting the hydrogen partialpressure to be 5 kPa or more, the oxygen (O) impurity concentration canbe significantly reduced to about 10¹⁶ cm⁻³ from about 10¹⁸ cm³. This isassumed because the porous surface of the thin film is activated byhydrogen and, therefore, the impurity to impair the transparency of thecrystal is likely to be trapped thereby. However, this effect can beobtained only before the surface of the substrate is covered with thegrown crystal. Namely, after the porous thin film is completely coveredwith the grown crystal, the effect will be eliminated.

(c) To Shorten the Growth Period with a Facet (i.e., a Flat CrystalPlane) Other than c-Plane to be Likely to Trap the Impurity

The growth of GaN is, in most cases, advanced on its initial stageaccording to a growth mode called Volmer-Waber type that a number ofthree-dimensional nuclei are generated and gradually combined each otherto form a continuous film. During the growth, facets such as (1-102) aregenerated on the side of a growth nucleus and are left as a pit for awhile even after the growth nuclei are combined each other.

The inventor finds that, in case of growing the crystal with the facets,the impurity, especially oxygen is likely to be trapped thereby ascompared to the case of growing the crystal with the flat c-plane (i.e.,substantially without any facets other than the c-plane). The oxygenthus trapped may be dispersed in the crystal to impair the transparencyeven after the growth surface is flattened. Thus, it is important tomake a transition to the flat growth as soon as possible.

FIG. 3 is a graph showing a relationship between a rate of the thicknessof a facet-grown layer in the entire thickness of a GaN layer and anoxygen concentration in the grown crystal.

As a result, it is understood that the oxygen impurity concentration canbe effectively reduced to a low range of 10¹⁶ cm⁻³ or less bycontrolling the thickness of the facet-grown layer in the entirethickness to be about 30% or less.

The method (c) is effective in reducing the impurity after the surfaceof the substrate is completely covered with the grown crystal.

The methods (a) to (c) may be conducted individually or in a combinationof any thereof.

Effects of the Embodiment

As described above, in the growth of GaN, by using any one or all of:

(a) the impurity trapping effect by the Ga melt;

(b) the impurity trapping effect by the porous film; and

(c) the early transition to growing with the plane less likely to trapthe impurity,

a nitride-based semiconductor substrate can be fabricated with:

(1) a high thermal conductivity of 1.2 W/cmK or more and 3.5 W/cmK orless, where the n-type carrier concentration is 1.2×10¹⁸ cm⁻³ or moreand 3×10¹⁹ cm⁻³ or less;

(2) a high electron mobility, μ [cm²/Vs], of more than a valuerepresented by log_(e) μ=17.7−0.288 log_(e) n and less than a valuerepresented by log_(e) μ=18.5−0.288 log_(e) n, where the n-type carrierconcentration n [cm⁻³] is 1.2×10¹⁸ cm⁻³ or more and 3×10¹⁹ cm⁻³ or less;

(3) a sufficiently low conductivity that a substrate electricalresistivity is 0.001 ohm·cm or more and 0.02 ohm·cm or less;

(4) a high crystalline quality that the dislocation density of thesubstrate is 1×10⁷ cm⁻² or less;

(5) a low impurity concentration that the concentrations of As, O, Cl,P, Na, K, Li, Ca, Sn, Ti, Fe, Cr and Ni are all 1×10¹⁷ cm⁻³ or less.

Thus, since the thermal conductivity is kept high while securing thepractically sufficient electrical conductivity (carrier concentration),the excessive heat can be fast dissipated as well as reducing theoperating voltage of the high-output device. Further, since the carriermobility is high, the sufficient electrical conductivity can be securedeven when doping at a lower concentration than the conventional method.Therefore, the thermal conductivity can be enhanced.

Furthermore, by fabricating a nitride-based semiconductor on thenitride-based semiconductor substrate thus made, a nitride-basedsemiconductor device such as LED and LD operated in large current can beproduced. Thus, the device can have a significantly enhanced operatingefficiency and lifetime.

Example 1 An Example to Use the Method (a)

FIG. 4 is a schematic diagram illustrating an HVPE reactor used inExample 1 according to the invention.

The HVPE reactor 10, which is a hot-wall type with a heater 2 outside ahorizontally long quartz reactor tube 1, comprises, on the left side(i.e., upstream side) of the quartz reactor tube 1, an NH₃ introducingtube 3 to introduce NH₃ gas, a group V source, an HCl introducing tube 4to introduce HCl gas for forming GaCl, a group III source, and a dopingtube 5 to introduce doping gas for controlling the conductivity.

The HCl introducing tube 4 is halfway enlarged in its inside diameter toprovide a Ga melt reservoir 6 to contain a Ga melt 7.

A substrate holder 9 with an underlying substrate 8 placed thereon isrotatably and movably disposed on the right side (i.e., downstream side)of the quartz reactor tube 1.

In growing GaN by using the HVPE reactor 10, the NH₃ gas as the group Vsource is introduced through the NH₃ introducing tube 3, the HCl gas toform the group III source through the HCl introducing tube 4, and adopant element-containing gas through the doping tube 5. Meanwhile, thesource gas, HCl gas and NH₃ gas are introduced mixed with a carrier gassuch as H₂ gas to control the reactivity.

In the HCl introducing tube 4, the HCl gas is halfway contacted with theGa melt 7 and thereby a reaction: Ga+HCl→GaCl+(½)H₂ is developed toproduce gallium chloride, GaCl.

In this process, the stay time of the HCl gas over the Ga melt 7 isadjusted to be 1 min. or more by controlling the flow rate of H2 carriergas based on a calculation in view of a volume of the Ga melt reservoir6. Thereby, the HCl gas and carrier gas can be purified.

The mixed gas of purified GaCl gas and H₂ carrier gas, and the mixed gasof NH₃ and H₂ carrier gas are conveyed to a direction as shown by arrowsin FIG. 4 in the space of the quartz reactor tube 1. Then, a reaction:GaCl+NH₃→GaN+HCl+H₂ is developed on the underlying substrate 8 placed onthe substrate holder 9 to deposit GaN on the underlying substrate 8. Inthe HVPE method, the GaN single crystal is epitaxially grown at anatmosphere temperature of about 800 to 1050 degree Celsius. Waste gas isremoved through a waste gas outlet (not shown).

The GaN single crystal thus grown can have a low impurity concentrationsince it is produced using the HCl source gas and H₂ carrier gas to bepurified by being contacted with the Ga melt with the high impuritytrapping effect for 1 min. or more.

Example 2 An Example to Use the Methods (a)+(b)+(c)

FIGS. 5A to 5G are cross sectional views showing a method of making aGaN self-standing substrate in Example 2 according to the invention.

A GaN self-standing substrate is made by a process as shown in FIGS. 5Ato 5G.

First, a sapphire substrate 11 with a diameter of 2 inches is providedas an underlying substrate (FIG. 5A). Then, a GaN film 12 is formed 300nm thick on the sapphire substrate 11 by MOVPE (FIG. 5B). Then, a Tifilm 13 is vacuum-deposited 20 nm thick (FIG. 5C) thereon, and thenheated at 1000 degree Celsius for 30 min. in a mixed atmosphere of H₂and NH₃ (with H₂ gas partial pressure of 80 kPa). By the heat treatment,the Ti film 13 on the surface of the substrate is nitrided such that itis changed into a porous TiN 14 with a number of microscopic pores withan inside diameter of tens of nanometers by the aggregation effect (FIG.5D).

Then, it is placed in the HVPE reactor 10 as shown in FIG. 4 and a GaNthick film 17 is grown 500 micrometers thick therein. In this process,the stay time of the HCl gas is adjusted to be 90 sec. by controllingthe flow rate of H₂ carrier gas based on a calculation in view of thevolume of the Ga melt reservoir 6. The H₂ partial pressure is 10 kPa,the GaCl partial pressure 2 kPa, and the NH₃ partial pressure 20 kPa.SiH₂Cl₂ is introduced through the doping tube 5 to adjust the final Siconcentration to 5×10¹⁸ cm⁻³ to secure a sufficient conductivity.

In the crystal growth process, a facet-grown GaN 15 is grown on theinitial stage (FIG. 5E) and then is combined each other to form acontinuous film. As the result of observing the section of a crystalgrown under the same conditions by a fluorescence microscope, thefacet-grown GaN 15 has a thickness of about 75 micrometers. Thus, therate of the thickness of the facet-grown GaN 15 in the entire thicknessof the GaN layer is about 15%.

A number of voids are formed on the interface of the porous film in theprocess of the HVPE growth (FIG. 5F). Therefore, the GaN thick film 17is by itself separated from the sapphire substrate 11 after the growth,whereby the GaN self-standing substrate 18 with a diameter of 2 inchesis obtained (FIG. 5G).

The dislocation density of the obtained GaN self-standing substratemeasured by the cathode luminescence method is a relatively good value,3×10⁶ cm⁻². The electrical resistivity thereof is a sufficiently lowvalue, 4×10⁻³ ohm·cm. O and Fe are not detected by SIMS analysis.Further, it is confirmed by the laser flash method that the thermalconductivity is as high as 2.0 W/cmK.

Comparative Example 1

FIGS. 6A to 6F are cross sectional views showing a method of making aGaN self-standing substrate in Comparative Example 1.

A GaN self-standing substrate is made by a process as shown in FIGS. 6Ato 6F.

First, a sapphire substrate 51 with a diameter of 2 inches is providedas an underlying substrate (FIG. 6A). Then, a GaN film 52 is formed 300nm thick on the sapphire substrate 51 by MOVPE (FIG. 6B). Then, a stripemask 53 of SiO₂ is formed thereon by photolithography (FIG. 6C). Themask width and the opening width are 15 micrometers and 10 micrometers,respectively.

Then, it is placed in the HVPE reactor 10 as shown in FIG. 4 and a GaNthick film 55 is grown 500 micrometers thick therein. In this process,the stay time of the HCl gas is adjusted to be 20 sec. by controllingthe flow rate of H₂ carrier gas based on a calculation in view of thevolume of the Ga melt reservoir 6. The H₂ partial pressure is 3 kPa, theGaCl partial pressure 0.5 kPa, and the NH₃ partial pressure 20 kPa.SiH₂Cl₂ is introduced through the doping tube 5 to adjust the final Siconcentration to 5×10¹⁸ cm⁻³ to secure a sufficient conductivity.

The crystal growth starts from the opening of the stripe mask 53, afacet-grown GaN 54 is then laterally grown (FIG. 6D), and a GaN thickfilm 55 is obtained with a flat surface (FIG. 6E). As the result ofobserving the section of a crystal grown under the same conditions by afluorescence microscope, the facet-grown GaN 54 has a thickness of about200 micrometers. Thus, the rate of the thickness of the facet-grown GaN54 in the entire thickness of the GaN layer is about 40%.

After the growth, the GaN thick film is separated from the sapphiresubstrate 51 by laser separation, whereby the GaN self-standingsubstrate 56 with a diameter of 2 inches is obtained (FIG. 6F).

The dislocation density of the obtained GaN self-standing substratemeasured by the cathode luminescence method is a relatively good value,8×10⁶ cm⁻¹. The electrical resistivity thereof is a sufficiently lowvalue, 4×10⁻³ ohm·cm. However, by SIMS analysis, O is detected as highas 1×10¹⁸ cm⁻² and Fe is detected about 3×10¹⁷ cm⁻². Further, it isconfirmed by the laser flash method that the thermal conductivity is aslow as 0.8 W/cmK.

Experiments

The GaN self-standing substrates obtained in Example 2 and ComparativeExample 1 are used to fabricate LED's with a same structure.

Both of them have the same optical output in 20 mA current feeding.However, in comparison of the optical output in 200 mA current feeding,it is found that the LED fabricated on the GaN self-standing substrateobtained in Example 2 is about 30% greater than that in ComparativeExample 1. This is assumed because the GaN self-standing substrate inExample 2 has a thermal conductivity higher than that in ComparativeExample 1 and, therefore, the excessive heat generated from the activelayer can be efficiently dissipated from the LED.

Example 3

(Relationship Between Carrier Concentration and Thermal Conductivity)

The GaN substrate is produced in the same manner as Example 2 to have acarrier concentration in the range of 1×10¹⁸ to 1.5×10¹⁹ cm⁻³.

FIG. 7 is a graph showing a relationship between a carrier concentrationand a thermal conductivity in Example 3 according to the invention. Forcomparison, the experiment results of Document 1 are shown therein.

As shown in FIG. 7, the thermal conductivity changes little in Example 3even when the carrier concentration is increased. In contrast, thethermal conductivity decreases significantly in Document 1 as thecarrier concentration is increased. At a practical carrierconcentration, the thermal conductivity is reduced to half or less thatat a low carrier concentration.

Comparative Example 2 LED Using a Sapphire Substrate

FIG. 8 is a schematic cross sectional view showing an LED structure inComparative Example 2. The LED in Comparative Example 2 is produced asbelow.

First, on a c-plane sapphire substrate 61 with a diameter of 2 inches, a4 micrometer-thick n-type GaN layer 62, a 40 nm-thick n-typeAl_(0.1)Ga_(0.9)N layer 63, an In_(0.15)Ga_(0.85)N/GaN-3-MQW activelayer 64 (well layer: 3 nm, barrier layer: 10 nm), a 40 nm p-typeAl_(0.1)Ga_(0.9)N layer 65, and a 500 nm-thick p-type GaN layer 66 aresequentially grown epitaxially by MOCVD. This epi-wafer is cut into 300micrometers square, and a p-type electrode 68 and an n-type electrode 67are formed on the top surface thereof and a surface of the n-type GaNlayer 62 exposed partially by dry etching.

The sapphire substrate-side of the chip is bonded through an Ag paste toa stem, and then an LED lamp is completed by wire bonding and resinsealing. The resultant LED lamp has a relatively low operating voltageof 4.2 V in 20 mA current feeding.

FIG. 12 shows current-output characteristics of Comparative Example 2(=COMP. EX. 2 in FIG. 12). The output increases in a low current regionup to about 20 mA as current increases. However, the output is almostsaturated in a current region higher than 20 mA, and the maximum outputis about 35 mW.

FIG. 13 shows a time dependency of relative output in 100 mA currentfeeding of Comparative Example 2 (=COMP. EX. 2 in FIG. 13), providedthat output at the start of the current feeding is 100%. The outputdecreases abruptly as time passes, and the lamp emits little light whenreaching about 100 hours or less. This is assumed because heat generatedfrom the active layer cannot be sufficiently dissipated due to the lowthermal conductivity of the sapphire substrate.

Comparative Example 3 LED Using a Conductive SiC Substrate

FIG. 9 is a schematic cross sectional view showing an LED structure inComparative Example 3. The LED in Comparative Example 3 is produced asbelow.

First, on an n-type SiC substrate 71 with a diameter of 2 inches, a 4micrometer-thick n-type GaN layer 72, a 40 nm-thick n-typeAl_(0.1)Ga_(0.9)N layer 73, an In_(0.15)Ga_(0.85)N/GaN-3-MQW activelayer 74 (well layer: 3 nm, barrier layer: 10 nm), a 40 nm p-typeAl_(0.1)Ga_(0.9)N layer 75, and a 500 nm-thick p-type GaN layer 76 aresequentially grown epitaxially by MOCVD. This epi-wafer is cut into 300micrometers square, and a p-type electrode 78 and an n-type electrode 77are formed on both surfaces thereof.

The chip is bonded through an Ag paste to a stem, and then an LED lampis completed by wire bonding and resin sealing. The resultant LED lamphas a relatively low operating voltage of 4.1 V in 20 mA currentfeeding.

FIG. 12 shows current-output characteristics of Comparative Example 3(=COMP. EX. 3 in FIG. 12). The output increases substantially linearlyup to a high current region as current increases due to the high thermalconductivity of SiC. However, the light extraction efficiency is lowsince the substrate is thickly colored into green. The output is, as awhole, not so high. The maximum output is about 50 mW.

FIG. 13 shows a time dependency of relative output in 100 mA currentfeeding of Comparative Example 3 (=COMP. EX. 3 in FIG. 13), providedthat output at the start of the current feeding is 100%. The output iskept nearly constant as time passes. This is assumed because heatgenerated from the active layer can be sufficiently dissipated due tothe high thermal conductivity of the SiC substrate.

Comparative Example 4 LED Using a GaN Substrate with a Low ThermalConductivity

FIG. 10 is a schematic cross sectional view showing an LED structure inComparative Example 4. The LED in Comparative Example 4 is produced asbelow.

First, a GaN self-standing substrate is prepared which has properties: acarrier concentration n and an electron mobility μ of n=5×10¹⁸ cm⁻³ andμ=100 cm²/Vs, respectively, measured by hole measurement; an electricalresistivity of about 0.01 ohm·cm; a thermal conductivity of 0.8 W/cmKmeasured by laser flash method; and a dislocation density of 2×10⁷ cm⁻²measured by cathode luminescence.

Then, on the GaN self-standing substrate 81 with a diameter of 2 inches,a 4 micrometer-thick n-type GaN layer 82, a 40 nm-thick n-typeAl_(0.1)Ga_(0.9)N layer 83, an In_(0.15)Ga_(0.85)N/GaN-3-MQW activelayer 84 (well layer: 3 nm, barrier layer: 10 nm), a 40 nm p-typeAl_(0.1)Ga_(0.9)N layer 85, and a 500 nm-thick p-type GaN layer 86 aresequentially grown epitaxially by MOCVD. This epi-wafer is cut into 300micrometers square, and a p-type electrode 88 and an n-type electrode 87are formed on both surfaces thereof.

The chip is bonded through an Ag paste to a stem, and then an LED lampis completed by wire bonding and resin sealing. The resultant LED lamphas a relatively low operating voltage of 4.1 V in 20 mA currentfeeding.

FIG. 12 shows current-output characteristics of Comparative Example 4(=COMP. EX. 4 in FIG. 12). The output increases substantially linearlyup to a low current region of about 40 mA as current increases. However,the output is saturated in a high current region, and the maximum outputis about 60 mW.

FIG. 13 shows a time dependency of relative output in 100 mA currentfeeding of Comparative Example 4 (=COMP. EX. 4 in FIG. 13), providedthat output at the start of the current feeding is 100%. The output isgradually reduced as time passes. At 500 hours, the output is reduced toabout 60% of the initial output. This is assumed because heat generatedfrom the active layer cannot be sufficiently dissipated due to theinsufficient thermal conductivity of the substrate though being higherthan that of the sapphire substrate. It is assumed that the low thermalconductivity is caused by that the doping amount must be increased toobtain a given electrical property due to the low electron mobility.

Example 4 LED Using a GaN Substrate with a High Thermal Conductivity

FIG. 11 is a schematic cross sectional view showing an LED structure inExample 4. The LED in Example 4 is produced as below.

First, a GaN self-standing substrate is prepared which has properties: acarrier concentration n and an electron mobility μ of n=2×10¹⁸ cm⁻³ andμ=330 cm²/Vs, respectively, measured by hole measurement; an electricalresistivity of about 0.01 ohm·cm; a thermal conductivity of 2.0 W/cmKmeasured by laser flash method; and a dislocation density of 3×10⁶ cm⁻²measured by cathode luminescence.

Then, on the GaN self-standing substrate 91 with a diameter of 2 inches,a 4 micrometer-thick n-type GaN layer 92, a 40 nm-thick n-typeAl_(0.1)Ga_(0.9)N layer 93, an In_(0.15)Ga_(0.85)N/GaN-3-MQW activelayer 94 (well layer: 3 nm, barrier layer: 10 nm), a 40 nm p-typeAl_(0.1)Ga_(0.9)N layer 95, and a 500 nm-thick p-type GaN layer 96 aresequentially grown epitaxially by MOCVD. This epi-wafer is cut into 300micrometers square, and a p-type electrode 98 and an n-type electrode 97are formed on both surfaces thereof.

The chip is bonded through an Ag paste to a stem, and then an LED lampis completed by wire bonding and resin sealing. The resultant LED lamphas a relatively low operating voltage of 4.0 V in 20 mA currentfeeding.

FIG. 12 shows current-output characteristics of Example 4. The outputincreases substantially linearly up to a current region of about 60 mAas current increases. Although the output is a little saturated in ahigher current region, and the maximum output is highest, about 95 mW.

FIG. 13 shows a time dependency of relative output in 100 mA currentfeeding of Example 4, provided that output at the start of the currentfeeding is 100%.

Significant reduction in output is little measured during this testperiod. This is assumed because heat generated from the active layer canbe sufficiently dissipated since the thermal conductivity of thesubstrate is sufficient to dissipate heat from the LED although beinglower than that of SiC, and because the internal quantum efficiency ishigh due to the low dislocation density and the light extractionefficiency is high due to the refractive index matching to theepi-layer. Further, it is assumed that the high thermal conductivity iscaused by that the doping amount is not needed to be increased much toobtain a given electrical property due to the high electron mobility.

Example 5 Relationship Between Carrier Concentration and ElectronMobility

GaN substrates are produced in the same manner as Example 2 to havecarrier concentrations of 2×10¹⁸, 4×10¹⁸, 6×10¹⁸, and 1.2×10¹⁹ cm³. Theelectron mobility μ is measured by hole measurement.

FIG. 14 is a graph showing a relationship between a carrierconcentration and an electron mobility in Example 5. For comparison,also shown therein are data as taught in Document 2 (S. Nakamura et al.,“Si- and Ge-doped GaN Films Grown with GaN Buffer layers”, Jpn. J. Appl.Phys. Vol. 31 (1992) pp. 2883-2888) and Document 3 (R. Y. Korotkov etal., “ELECTRICAL PROPERTIES OF OXYGEN DOPED GaN GROWN BY METALORGANICVAPOR PHASE EPITAXY”, MRS Internet J. Nitride Semicond. Res. 5S1, W3.80(2000)).

As shown in FIG. 14, the electron mobility in Example 5 falls within aregion (shaded region) surrounded between Formula A (log_(e)μ=17.7−0.288 log_(e) n) and Formula B (log_(e) μ=18.5−0.288 log_(e) n),where the carrier concentration thereof is in the range of 1.2×10¹⁸ cm⁻³or more and 3×10¹⁹ cm⁻³ or less. It is proved that the electron mobilityin Example 5 is higher than that in Documents 2 and 3.

Other Applications and Modifications

Although in the above embodiments the invention is applied to the LED,the invention can be also applied to a high-output device such as LD andpower conversion device other than the LED, whereby the same effects canbe obtained.

Although the invention has been described with respect to the specificembodiments for complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. A nitride-based semiconductor substrate,comprising: a substrate comprising a nitride-based semiconductor,wherein the substrate comprises: a diameter of 25 mm or more; athickness of 250 micrometers or more; a dislocation density of 1×10⁷cm⁻² or less; an oxygen impurity concentration of 10¹⁶ cm⁻³ or less; ann-type carrier concentration in a range of 1.2×10¹⁸ cm⁻³ to 3×10¹⁹ cm⁻³;and a thermal conductivity in a range of 1.2 W/cmK to 3.5 W/cmK.
 2. Thenitride-based semiconductor substrate according to claim 1, wherein: thesubstrate comprises an electrical resistivity of 0.001 ohm·cm or moreand 0.02 ohm·cm or less.
 3. The nitride-based semiconductor substrateaccording to claim 1, wherein the substrate comprises a concentration ofAs, Cl, P, Na, K, Li, Ca, Sn, Ti, Fe, Cr and Ni to be all 1×10¹⁷ cm⁻³ orless.
 4. A semiconductor device, comprising: the nitride-basedsemiconductor substrate as defined in claim 1; and a nitride-basedsemiconductor layer grown epitaxially on the nitride-based semiconductorsubstrate.